A selective adaptation procedure will be used to study the detailed properties of feature detection mechanisms involved in the perception of speech sounds. This procedure consists of three steps: (1) a series of test stimuli which vary along some acoustic (and phonetic) dimension are first presented to subjects for identification (or discrimination), (2) next, an adapting stimulus is presented repeatedly for a period of one or more minutes, (3) following adaptation, the original test stimuli are again presented for identification (or discrimination). Previous studies have shown that adaptation with a speech sound having a particular feature (e.g., voiced) produces a reduction in the number of test stimuli identified as having that feature. Current accounts suggest that such an alteration in identification performance results from the fatigue (and hence reduction in sensitivity) of the relevant feature detectors during adaptation. The proposed research will attempt to address three general questions: What are the relative contributions of sensory-perceptual and response factors to the overall adaptation effect? What are some of the detailed properties of acoustic feature detectors involved in speech perception? Is there a level of phonetic feature extraction in speech perception which is distinct from the level of acoustic feature analysis?